- dest[1] = src[x] - src[x-4];
- dest[2] = src[x] - src[x-srcwidth];
- dest[3] = src[x] - (src[x-4] + src[x-srcwidth])/2;
- dest[4] = src[x] - PaethPredictor(src[x-4], src[x-srcwidth], src[x-4-srcwidth]);
+ dest[1] = src[x] - src[x-back_x];
+ dest[2] = src[x] - src[x-back_y];
+ dest[3] = src[x] - (src[x-back_x] + src[x-back_y])/2;
+ dest[4] = src[x] - PaethPredictor(src[x-back_x], src[x-back_y], src[x-back_x-back_y]);